Teghtsoonian et al (Perception and Psychophysics, 2000) report that magnitude estimates for loudness of a very weak tone are roughly 10-fold lower when presented at the end of a slowly and continuously decreasing intensity sweep compared to a presentation at the beginning of a continuously increasing intensity sweep. This decruitment effect was also reported for visual perceived size of a very small disk. Neuhoff (Nature, 1998) proposed that decruitment of loudness and perceived size may promote attention to approaching rather than receding targets. We reported decruitment effects for perceived contrast last year (Stephens and Dannemiller, 2002). Observers provided magnitude estimates of perceived contrast for gratings whose contrast increased or decreased logarithmically over a 45 sec sweep. Estimates for the low contrast probes were two-fold lower in the decreasing sweep conditions.

Since magnitude estimation procedures may be sensitive to subtle methodological variables, we examined contrast decruitment using a contrast matching procedure. Stimuli were stationary squarewave gratings. In each trial, the grating's contrast was constant, increased, or decreased logarithmically over a 45 sec sweep, with start or end points of 0.2 and 0.006 contrast. A probe contrast (0.1, 0.07, 0.01, or 0.007) was cued by a tone during each sweep. Observers (n=17) adjusted the contrast of a comparison grating to match the perceived contrast of the probe contrast after each sweep. For all probe contrasts, match estimates were lower (p<0.05) in the decreasing compared to the increasing or constant sweep conditions. In a second experiment, timing cues were removed by ending the sweeps immediately after signaling probe contrasts. No decruitment was observed: match estimates were virtually identical in the three sweep conditions. These results suggest that contrast decruitment effects may be explained by response bias.